I'm guessing a key point is the flat panel on top of the radiator and it's slightly elevated front portion.
My only thought is the big flat spot of the bed is an area of detached air flow and you might not get as much flow into the radiator as you would think. Either the small scoop effect of the top at the back of his bed or a small spoiler might create the high pressure area you want. Maybe some real aero guys would know.
